Nonwoven

ABSTRACT

The present invention relates to a nonwoven that includes superposed plies of fibers, such that a binder web is positioned between two plies and heat sealed to the at least two plies, and that it is not stitched. The heat sealing of the plies makes the nonwoven extremely solid and sets the orientation of the fibers, which can no longer move during the processing of the nonwoven. The fibers are, in fact, held by weld points that are non-continuous and are distributed over the entire surface, unlike stitching.

This application claims priority to French patent Application No.1560839 filed on Nov. 12, 2015, the entire contents of which areincorporated herein by reference.

BACKGROUND

The present invention relates to a nonwoven composed of severalsuperposed fiber plies.

The holding together of these various plies may pose a problem,especially if the nonwoven is used for producing pultruded profiles.

This problem is even more critical when the plies of fibers consist offibers of different orientations, specifically if the pultrusion processis used for producing profiles with unidirectional fibers in the lengthdirection of the profile, the orientation of the fibers is unlikely tobe modified, but when the fibers have an orientation different from thedirection of the pultrusion operation they have a tendency to move in anuncontrollable manner.

In particular, if it is desired to reinforce the profiles in the otherdirections, such as for example at 45° and/or at 90°, use is made ofcommercially available multiaxial reinforcements which are producedeither by weaving, or by laying down of plies stitched together in orderto form multiaxial nonwovens, in this case, the solidity (cohesion andmaintaining of the angles during the processing) of the nonwoven dependson the stitching. These multiaxial reinforcements have the drawback ofbeing fragile during their processing and particularly for theprocessing by pultrusion where it is necessary to pull on the multiaxialreinforcements and make them pass through a die. In particular, thesemultiaxial reinforcements become distorted and when the angles areinitially 45° and 90°, these angles are no longer maintained after theprocessing. The mechanical characteristics of these profiles are thengreatly reduced. Moreover, when there are no fibers in the lengthdirection)(0°) in the nonwoven the tensile strength decreases greatlyand is very low in the case where there are only fibers oriented at +and −45°. This may be particularly critical for certain applicationssuch as aeronautics.

The yarns are bound at the stitch point but may move easily between thestitch points (spaced for example 5 to 10 mm apart), since there are fewbonding points between the fibers. In addition, the stitches give riseto a deformation of the fiber ply to allow the needle to pass through,thus creating a hole which may be annoying on the one hand if a certainsurface protection is desired for the possible electrical performanceand/or the surface homogeneity, and on the other hand which may misalignthe fibers leading to losses of the mechanical characteristics.

It is possible to join plies of transverse yarns by welding by means ofa grid, these yarns being bound together by binding yarns to enable tobe kept yarns parallel one another in the same plies, however the weldpoints will then be distributed in accordance with the grid, that is tosay spaced apart according to a predetermined distance, thus the hold ofthe yarns will be insufficient.

In order to produce preformed parts, it is known to use a multiaxialfabric comprising reinforcing layers of unidirectional fibers withnonwoven intermediate layers comprising a spunbond or spunlace fabric ormesh of thermoplastic fibers positioned between the reinforcing layers.This process does not make it possible to guarantee the maintaining ofthe direction of the fibers during preforming or forming operations.

It is also possible to pre-impregnate the reinforcing fibers in order togive them an ability to better maintain their alignment than the sameunimpregnated fibers leading to a significant increase in the productioncosts and the complexity of the processing (transport and storagecondition, storage and operating temperature).

The fibers may also be covered with thermoplastic or thermosettingpowder in order to bind the yarns by points. However, the use of powdermakes it necessary to calibrate the amount of product to use and toeliminate the surplus.

SUMMARY

The invention proposes to produce nonwovens that eliminate thesedrawbacks, a process for producing this type of nonwoven and also theprofile obtained with this nonwoven.

The nonwoven according to the invention consists of superposed plies offibers, it is characterized in that a binder web is positioned betweentwo plies and heat sealed, and that it is not stitched. The heat sealingof the plies makes the nonwoven extremely solid and sets the orientationof the fibers which can no longer move during the processing of thenonwoven. The fibers are, in fact, held by weld points that arenon-continuous and are distributed over the entire surface, unlikestitching. The fact that the nonwoven is not stitched makes it possibleto guarantee a constant orientation of the fibers with no discontinuity.

Advantageously, the nonwoven is a pultrusion nonwoven. This nonwoven isparticularly suitable for pultrusion since it is not pierced bystitching and has a dense distribution of weld points of the fiberswhich are thus particularly well immobilized.

Advantageously, the plies consist of oriented fibers. The heat sealingmakes it possible to have more bonding points between the fibers andtherefore to guarantee a better hold of these fibers in particular whenthese fibers are oriented.

Advantageously, two superposed plies have different fiber orientations.The profiles have a better strength if nonwovens with fibers of oppositeorientations are used.

Advantageously, the binder web is a thermoplastic. The advantageprovided by these thermoplastic webs is that of improving the impactresistance of the composite obtained.

According to one particular arrangement, the nonwoven comprises asurface web. The heat sealing of a surface web (which may be a glass mator a polymer web) makes it possible to provide functions of protection(against galvanic corrosion, against abrasion, etc.) during theproduction of profiles made of carbon. In general, for a betterprotection, or even a better insulation, the surface web should not bepierced and cannot therefore be stitched.

Advantageously, the binder web has a melting point above a pultrusiontemperature. The binder web, which may be a thermoplastic, will bechosen to have a melting point above the temperature used during thepultrusion process so as to avoid a degradation of the nonwoven duringthe pultrusion operation.

Advantageously, the binder web has a basis weight less than or equal tothe basis weight of the fibers of the plies. The nonwoven should remainpermeable in order to enable the pultrusion. The basis weight of thebinder webs or of the thermoplastics should be low enough to enable agood impregnation of the fibers but high enough to avoid any movement ofthe fibers after heat sealing. The basis weight of the binder web or ofthe thermoplastic depends on the basis weight of the fibers; it shouldbe less than 10% of the basis weight of the fibers, preferably less than5%. For example, a basis weight between 4 and 8 g/m² makes it possibleto heat seal plies of 100 to 300 g/m² of carbon.

Advantageously, certain yarns are reinforcing fibers. These fibers maybe made of carbon, of glass fibers, of aramid (Kevlar™), etc.

The invention also relates to a process for producing a nonwoven thatconsists in superposing plies of fibers and at least one binder webbetween two plies of fibers, then in heat sealing the assembly.

Advantageously, a surface web is positioned on the plies of fibersbefore the heat sealing. The surface web should be counted in the layersto be taken into account during the heat sealing operation. Thereinforcing fiber layers are all heat sealed in a single go. However itis possible to repeat this operation several times in order to attainthe desired stack of plies, for example a reinforcing web may thus beheat sealed to a pre-existing multiaxial reinforcement.

The invention also relates to a pultruded profile comprising a nonwovenwith at least one of the preceding features. The pultruded profile usesa nonwoven obtained according to the process. The profile thus obtainedis particularly suitable for aeronautical uses which are particularlydemanding.

Other advantages may also become apparent to those skilled in the art onreading the examples below, illustrated by the appended figures, givenby way of example:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a stitched nonwoven according to the prior art,

FIG. 2 is a schematic view of a nonwoven according to the invention,

FIG. 3 is a top view of a nonwoven according to the invention.

DETAILED DESCRIPTION

The nonwoven from the prior art illustrated in FIG. 1 here consists offour plies 1, 2, 3 and 4 of fibers 10, 20, 30 and 40 that are superposedand stitched together by stitching 5. The fibers of a same ply areparallel to one another. Here, the plies are oriented differently oneach layer.

It will be considered that the vertical fibers in FIG. 1 are in the pulldirection, that is to say that they make an angle of 0° . The fibers 20of the ply 2 therefore make an angle of 90°. The fibers 20 of the secondply 2 are positioned on the fibers 10 of the first ply 1 oriented withan angle of 45°. The third ply 3 is positioned on the second ply 2 andits fibers 30 are positioned with an angle of −45° relative to thefibers 40. The fourth ply 4 has fibers 40 positioned at 0°, i.e. alongthe length of the nonwoven.

The stitching 5 creates bonding points between the plies in a linear andspaced out manner. The stitching 5′ is more sinuous. The fibers arebound at the stitch point but may move easily between the stitch pointswhich are in general spaced 5 to 10 mm apart.

In this figure, the nonwoven is quadraxial (with angles of 45°, 90°,−45° and 0°) and its solidity depends on the stitching. But when thereis no 0° in the nonwoven, the tensile strength decreases greatly and isvery low in the case where there is only +and −45°.

In FIG. 2, there are three oriented plies 1, 2, 3: the ply 2 consists offibers 20 having an angle of 0° , whereas the plies 1 and 3 consist offibers 10 and 30 having angles of +or −45°. Each ply is separated fromthe next by a binder web 6.

For example, in order to fix a four-ply nonwoven a single binder webbetween two plies is sufficient, and in order to fix the other plies itis necessary either to insert binder webs or to add stitching as in theprior art.

The nonwoven illustrated in FIG. 3 consists of two plies 1 and 3oriented at + or −45°, and an inserted binder web 6. As can be seen inthe figure, this web 6 which will be heat sealed, covers a large portionof each ply 1 and 3, there will therefore be a large number of bondingpoints of the fibers of each ply rendering the various layers firmlyattached.

The web 6 will preferably be very thin in order to make it possible toproduce a multitude of micro-bondings preventing the fibers 10 and 30from moving.

In this example, the nonwoven has only two layers (2 plies) but it ispossible to add other layers by (optionally) inserting other binder webs6.

The principle remains the same if the last layer is replaced by asurface web.

1. A nonwoven comprising at least two superposed plies of fibers; abinder web positioned between the at least two plies and being heatsealed, wherein the nonwoven is not stitched.
 2. The nonwoven accordingto claim 1, wherein it is a pultrusion nonwoven.
 3. The nonwovenaccording to claim 1, wherein the plies comprise oriented fibers.
 4. Thenonwoven according to claim 1, wherein two superposed plies havedifferent fiber orientations.
 5. The nonwoven according to claim 1,wherein the binder web is a thermoplastic.
 6. The nonwoven according toclaim 1, further comprising a surface web.
 7. The nonwoven according toclaim 2, wherein the binder web has a melting point above a pultrusiontemperature.
 8. The nonwoven according to claim 3, wherein the binderweb has a basis weight less than or equal to the basis weight of thefibers of the plies.
 9. The nonwoven according to claim 3, whereincertain fibers are reinforcing fibers.
 10. The nonwoven according toclaim 1, wherein the plies comprise carbon fibers.
 11. A process forproducing a nonwoven comprising superposing plies of fibers and at leastone binder web between two plies of fibers to form an assembly; and,heating sealing the assembly.
 12. The process according to claim 11,further comprising positioning a surface web on the plies of fibersbefore heat sealing.
 13. A pultruded profile comprising a nonwovencomprising at least two superposed plies of fibers; a binder webpositioned between the at least two plies and being heat sealed, whereinthe nonwoven is not stitched.